A researcher at IBM's Watson Research Center has a new idea about how to slam spam: Make spammers pay to send messages. Scott Fahlman has written a basic algorithm that can determine whether incoming E-mails hail from an addressee on a recipient-defined list of approved addresses. Messages from addresses not on the list would be scoured for a 10-digit authentication code obtained from software running the algorithm or a "charity stamp" site that would issue such codes for a fee small enough to be acceptable to legitimate E-mailers; messages without the codes would be returned to the sender. Says Fahlman, "If we change the social rules of E-mail just a tiny bit, I think the whole problem of spam goes away."